Statement of problem
Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) restorations of different formulations are being used increasingly in monolithic form for a range of clinical applications. Using rotary instruments to remove such restorations for any purpose is challenging, but they can be removed conservatively with erbium lasers. However, studies on how a laser penetrates different zirconias to break the cement bond between the tooth surface and the zirconia are lacking.

Purpose
The purpose of this in vitro study was to evaluate and compare the time required for an erbium-doped yttrium-aluminum-garnet (Er:YAG) laser to remove different types of zirconia and lithium disilicate crowns.

Material and methods
Forty extracted premolar teeth were prepared, scanned, designed, and milled to fabricate 40 computer-aided design and computer-aided manufacturing (CAD-CAM) crowns, which were assigned to 4 groups (n=10): 3 mol% Y-TZP, 4 mol% Y-TZP, 5 mol% Y-TZP, and lithium disilicate as a control. All crowns were bonded to the teeth with a self-adhesive resin cement (Panavia SA Cement Universal). Each specimen was irradiated with an Er:YAG laser with the following parameters: 300 mJ, 15 Hz, 5.0 W, and a 50-microsecond pulse duration (supershort pulse mode). The irradiation time required for crowns to be retrieved successfully was recorded for each specimen. Data were statistically analyzed using analysis of variance and the Tukey honestly significant difference post hoc test (α=.05). The intaglio surfaces of the restorations were analyzed using scanning electron microscopy (SEM).

Results
The mean ±standard deviation times in minutes needed for crown debonding were 12.46 ±4.17 for the 3 mol% Y-TZP group, 10.30 ±3.33 for the 4 mol% Y-TZP group, 4.03 ±1.62 for the 5 mol% Y-TZP group, and 2.08 ±0.92 for the lithium disilicate group. A statistically significant difference (P<.05) in the debonding time was found for all investigated groups, expect between the 3 mol% and 4 mol% Y-TZP groups and between the 5 mol% Y-TZP and lithium disilicate groups. SEM analysis of the ceramic surfaces showed no visual damage associated with Er:YAG laser irradiation.

Conclusions
Zirconia crown retrieval time with the Er:YAG laser was influenced by the yttria content of the zirconia, with decreasing retrieval time with increasing yttria content. Er:YAG laser debonding of zirconia crowns is a noninvasive, efficient, and rapid approach to the removal of crowns and could be applied in clinical practice.